The present invention relates to air cleaners and more particularly to portable air cleaners which employ permeable replacement filters through which air is passed for trapping and removing particulates from the air stream.
In air cleaners of the class described, the filters are generally in sheet-like form with fibrous material or tiny pellets presenting extremely small tortuous passageways or openings for the flow of air through the filters. In passing through the filters, particulates in the air are trapped within the filters and the treated air minus the particulates is discharged from the air cleaning device. In operation, a buildup of particulates on the filters occurs and in due course the filters become increasingly clogged, reducing the amount of particulates per unit volume of air being removed from the air stream passing through the filters. Generally, a motor driven fan is utilized to draw the air through the filters and as the filters become more clogged, the pressure of the air downstream of the filters becomes less so that a differential pressure exists across the filters which gradually becomes greater as the permeability of the filters is reduced. Eventually, the effectiveness of the filters in reducing particulates becomes minimal and the clogged filters must be replaced, to return the air cleaner to an efficient operating condition.
In present portable air cleaners, the air cleaners may be operated by users long past the time when the filters should be replaced because the filters have substantially lost their effectiveness in treating the air. One of the reasons for lack of action by the user is that in these appliances the change in flow of air discharged from the housing, when the filters are clogged, is not appreciably different from the air flow when the filter is not clogged or partly clogged. Because the user is unable to sense the difference in air flow the filters become so clogged as to render them ineffective for the purposes for what they are intended. There are many users of these appliances also that forget, or even perhaps, do not know that the filters should be replaced. As a result, one of the principal purposes of the appliance is defeated because the user is not alerted to the replacement need. If the condition continues, the user may feel that the air cleaner is improperly operating and be tempted to set it aside and not use the appliance. Or, it may become a cause of complaint to the manufacturer who may have to expend needless time and expense to satisfy the customer--user.